When you install the package using the RStudio package installer or directly from CRAN, it doesn’t install the dependencies (“fracdiff”, “Rcpp”, “RcppArmadillo” and “colorspace”) and hence, R keeps throwing the load namespace error. Installing the package through, automatically installs all the dependencies and solves this problem.
install.packages("forecast",
repos = c("http://rstudio.org/_packages",
"http://cran.rstudio.com"))